Salt separation and destruction of PFAS utilizing reverse osmosis and salt separation

ABSTRACT

Per- and polyfluoroalkyl substances (PFAS) are destroyed by oxidation in supercritical conditions. PFAS in water is concentrated in a reverse osmosis step and salt from the resulting solution is removed in supercritical conditions prior to destruction of PFAS in supercritical conditions.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/396,599, filed 6 Aug 2021 and claims priority to U.S. ProvisionalPatent Application Ser. No. 63/062,251 filed 6 Aug. 2020.

INTRODUCTION

Per- and polyfluoroalkyl substances (PFAS), including perfluorooctanesulfonate (PFOS) and perfluorooctanoic acid (PFOA), and hundreds ofother similar compounds, have been widely used in the United States in amultitude of applications. There are significant concerns associatedwith these compounds due to widespread contamination coupled withuncertainties about risks to human health and the environment. PFAS aremolecules having chains of carbon atoms surrounded by fluorine atoms.The C—F bond is very stable enabling the compounds to persist in thenatural environment. Some PFAS include hydrogen, oxygen, sulfur,phosphorus, and/or nitrogen atoms. One example is PFOS:

Although some PFAS compounds with known human health risks have beenvoluntarily phased out (PFOA and PFOS), legacy contamination remains.Replacement PFAS compounds have been introduced with limitedunderstanding of their health risks. Currently, only PFOA and PFOS areaddressed in Lifetime Health Advisories at the Federal level, with noestablished maximum contaminant level (MCL) to regulate the acceptablelevel of these and other PFAS compounds in drinking water. PFAScontamination in drinking water sources in 1,582 locations in 49 statesas of May 2020. Currently used techniques for treating PFAS-contaminatedwater are expensive, and management of spent media is costly and mayresult in long-term liability.

Numerous methods have been developed for remediating PFAS in theenvironment. For example, Oberle et al. in US 2019/0314876 describes amethod and system for remediating soil containing PFAS in which the soilis heated and the PFAS volatilized, captured and condensed, steam added,and then the concentrated PFAS solution subjected to electro oxidation.

Application of SCWO to PFAS is relatively new and presents newchallenges. SCWO of organic compounds has long been known and isdescribed in numerous papers and patents. For example, Welch et al. inU.S. Pat. No. 4,861,497 described the use of a liquid phase oxidant suchas hydrogen peroxide or ozone in supercritical water for the destructionof organic compounds; testing with destruction of propylene glycol at750 to 860° F. at 5000 psia (pounds per square inch atmospheric)resulted in about 98% destruction. Swallow et al. in U.S. Pat. No.5,232,604 described SCWO of organic compounds with an oxidant such ashydrogen peroxide and a reaction rate enhancer such as nitric oxide; inone example, sodium hydroxide and sodium nitrate were used to neutralizehydrochloric acid formed in the oxidation of methylene chloride.Aquarden Technologies in US Published Patent Application No.2019/0185361 notes that in the SCWO process precipitation occurs in azone where the fluid goes from sub-critical to super-critical anddesigned a reactor with a residue outlet connection near this zone.Miller et al. in “Supercritical water oxidation of a model fecal sludgewith the use of a co-fuel” Chemosphere 141 (2015) 189-196 reported onthe SCWO reaction of a feces simulant in the presence of 48% excessoxygen. The use of auxiliary fuels can be used to generate hydrothermalflames in SCWO reactors that are characterized by high temperatures,typically above 1000° C. See “Supercritical Water Oxidation,” inAdvanced Oxidation Processes for Wastewater Treatment,” (2018), 333-353.

Despite extensive prior efforts to develop systems for destroying PFAS,there remains a need for efficient systems for treating PFAScompositions and the complete destruction of PFAS.

SUMMARY OF THE INVENTION

Initially, water or soil samples may be treated to concentrate PFAS in asubstantially reduced volume. In some instances, the PFAS-containingmedia has been stored in a concentrated form and does not requireadditional treatment to concentrate it. The concentrated PFAS mixturescan be put in containers and shipped to a centralized site for PFASdestruction. Alternatively, in some preferred embodiments, theconcentrated PFAS mixtures are treated on-site where they originate. Theconcentrated PFAS solution is destroyed by Supercritical Water Oxidation(SCWO), which we have found can rapidly result in over 100,000 timesreduction in PFAS concentration, for example, a reduction in PFOA from1700 parts per million (ppm) to 5 parts per trillion (ppt) by weight orless. To enable efficient destruction with little or no external heatsupply during steady state operation, fuels may be added (or, in someoccurrences, PFAS may be present with sufficient organic materials thatserve as the fuel) to supply some or all of the heat needed to power theoxidation. The resulting effluent can then be confirmed to containlittle or no PFAS, typically 5 ppt or less, and then be released backinto the environment as safe, clean water.

In one aspect, a PFAS-containing aqueous solution is subjected to one orpreferably a cascade of plural stages of reverse osmosis to form a brineand a desalted solution. Prior to reverse osmosis (RO), thePFAS-containing aqueous solution is typically subjected to optionalfiltration and one or more water softening steps in which ions such asCa and Mn are replaced by Na ions. The net result of the cascade ispreferably at least 60%, more preferably at least 80% reduction, and insome embodiments 80 to 95% reduction in volume of the fraction that issubsequently subjected to processing including SCWO to destroy the PFAS.The PFAS is partitioned so that at least 80%, preferably at least 90% orat least 95% or at least about 99% of the PFAS in the initial aqueousmixture is partitioned into the brine fraction resulting from thereverse osmosis process.

In another aspect, the invention provides a method of destroying PFAS,comprising: providing an aqueous solution comprising water and PFAS;subjecting the aqueous solution to reverse osmosis to produce a cleanwater fraction and a briny concentrated fraction in which the PFASconcentration is at least 50% greater than the aqueous solution;preheating the briny concentrated fraction in a heat exchanger to form apreheated concentrated fraction that is at subcritical conditions;passing the preheated concentrated fraction into a heated pre-reactorwhere the briny concentrated fraction is converted to supercriticalconditions at a first temperature causing sodium chloride toprecipitate; removing at least a portion of the sodium chloride toproduce a brine-reduced fraction; passing the brine-reduced fraction toa reactor where the fraction is subjected to oxidation undersupercritical conditions wherein the concentration of oxidant and/ortemperature is higher than in the pre-reactor; producing a clean hotwater solution having a concentration of PFAS that is at least 90% lessthan the aqueous solution; and, optionally, transferring heat from theclean hot water solution to the aqueous solution in the heat exchangerin the preheating step.

The method can be further characterized by one or any combination of thefollowing characteristics: wherein the aqueous solution is filteredprior to reverse osmosis; wherein the effluent passes through a postheat exchanger that is positioned downstream of the pre-reactor and thenpasses through a pre heat exchanger that is positioned upstream of thepre-reactor (the direction of flow (upstream or downstream) refers tothe flow of the aqueous solution being subjected to PFAS destruction);wherein the briny concentrated fraction in which the PFAS concentrationis at least 50% greater, that results from the reverse osmosis and priorto the step of removing at least a portion of the sodium chloride,comprises a precipitate and further comprising a step of adding an acidto dissolve the precipitate; wherein the pre-reactor comprises a firsttube leading into a collector vessel and a second tube passing out ofthe collector vessel, wherein the collector vessel comprises an innerdiameter that is at least twice as large as the inner diameter of thefirst tube; and, optionally, wherein a fuel or oxidizer is added to thepre-reactor; wherein the pre-reactor comprises a transcriticalhydrocyclone, preferably of the type described herein.

The invention provides an energy efficient method of destroying PFASthat is optionally characterized by the steps of reverse osmosis andsalt separation followed by oxidation in a SCWO reactor; and optionallycharacterizable wherein the aqueous solution comprising water and PFAShas a first volume; wherein 10% or less (or 5% or less, or 1% or less,or 0.1% or less) of the first volume is subjected to supercriticalconditions; and wherein, in said method, at least 95% (or at least 98%or at least 99%) of the PFAS in the first volume is destroyed insupercritical conditions.

In another aspect, the invention comprises a system for destroying PFAS,comprising: a reverse osmosis system; a conduit from the reverse osmosissystem to a salt separator; wherein the salt separator comprises a firsttube leading into a collector vessel and a second tube passing out ofthe collector vessel, wherein the collector vessel comprises an innerdiameter that is at least twice as large as the inner diameter of thefirst tube; and a conduit from the salt separator to a supercriticalreactor.

The system can be further characterized by one or any combination of thefollowing characteristics: wherein the collector vessel comprises asupercritical fluid; wherein the first tube projects at least 5 cm intothe collector vessel and the second tube does not project into theinterior of the collector tube; further comprising a first heatexchanger disposed in the conduit between the reverse osmosis system andthe salt separator; wherein, during operation, the first heat exchangerexchanges heat between a subcritical PFAS-containing aqueous stream anda return stream of PFAS-free water from the supercritical reactor;wherein the first tube projects at least 5 cm further into the collectorvessel than the second tube; wherein the heat exchanger is atube-in-tube heat exchanger; wherein the salt separator comprises aplurality of collector vessels each collector vessel comprising a set ofan inlet tube and an outlet tube; wherein each set is at a highertemperature that the previous set in the direction of flow.

In a further aspect, the invention provides a transcriticalhydrocyclone, comprising: a conical chamber comprising: an inlet forintroducing supercritical fluid into the conical chamber tangentiallyalong an inner wall of the cyclone; a top outlet adapted for flow of asupercritical fluid; an exit pipe adapted for flow of a liquid; a conedisposed in the conical chamber adapted such that a channel can beformed between the inner wall of the cyclone and an outer wall of thecone; and wherein the cone comprises a bottom opening.

The hydrocyclone can be further characterized by one or any combinationof the following characteristics: wherein the bottom opening comprises adiffuser—diffuser comprising a plurality of openings; wherein theplurality of openings comprise an open area that is at least two timesor at least three times as large as the cross-sectional area of the exitpipe; wherein the openings are angled (not in a straight line (i.e., notthe shortest distance)) from the central axis to an inner wall of thelower pipe; further comprising a mechanism adapted to move the conealong the central axis of the conical chamber relative to the inner wallof the conical chamber; comprising heat exchanger thermally connected toan outer wall of the conical chamber; wherein the heat exchangercomprises a jacket for fluid flow.

The invention also provides apparatus having any of the passivatingcoatings described herein. The invention further provides a method ofdestroying PFAS, comprising: providing an aqueous solution comprisingwater and PFAS; preheating the aqueous solution; providing a reactorcomprising reactor walls coated with a ceramic coating that is resistantto hydrofluoric acid; treating the solution in the reactor with anoxidant under conditions in which water is in the supercritical phase toproduce a clean hot water solution having a concentration of PFAS thatis at least 90% less than the aqueous solution; and transferring heatfrom the clean hot water solution to the aqueous solution in a heatexchanger in the preheating step. Preferably, the ceramic comprises: B4C(boron carbide), SiC (silicon carbide), TaC (tantalum carbide), WC(tungsten carbide), metal fluorides such as YF3 (yttrium fluoride), YN(yttrium nitride), LaF3 (lanthanum fluoride), LaN (lanthanum nitride),YbN, YbF3, or any lanthanide nitride or lanthanide fluoride, HfN(hafnium nitride), CeN (cerium nitride), CeF3 (cerium fluoride), TaN(tantalum nitride), Ta (tantalum), TaF (tantalum fluoride), ZrN(zirconium nitride), ZrF (zirconium fluoride), WN (tungsten nitride),chromium oxide, or combinations thereof.

Any of the inventive aspects may be further defined by one or anycombination of the following: wherein the method is carried out in amobile trailer; the PFAS-containing aqueous mixture comprises at least100 ppm PFAS and the method decreases the PFAS concentration by at least10⁶ or 10⁷ or 10⁸; wherein the PFAS is reacted with oxidant in anoxidation reactor and after leaving the reactor the effluent is treatedwith a solution comprising NaOH, LiOH, or KOH to produce a neutralizedsolution that can be discharged or recycled to neutralize additionaleffluent; wherein the neutralized effluent is at least partiallyevaporated into the air; wherein by taking a PFAS-concentration whereinthe PFAS-containing aqueous mixture comprises at least 100 ppm PFAS byweight (in some embodiments at least 500 ppm or at least 1000 ppm PFAS)that the method converts to an effluent comprising 1 ppm or less, or 0.1ppm or less, or 0.01 ppm or less, or 1.0 ppb or less, or 0.1 ppb orless, or 0.01 ppb or less PFAS; in some embodiments in the range of 1ppm to 5 ppt (part per trillion) PFAS or less; wherein thePFAS-containing solution is mixed with a solution comprising 30 to 50 wt% H₂O₂ at a weight ratio of preferably 30:1 to 70:1 wt % ratio PFASsolution:H₂O₂; wherein the PFAS-containing solution is passed through aSCWO reactor with a residence time of 20 sec or less, preferably 10 sec,or 5 sec or less, or 0.5 to 5 seconds; wherein the PFAS-containingsolution is added at a rate controlled between 50 and 150 mL/min (atSTP); wherein no external heating is required after start-up; whereinthe PFAS-containing aqueous mixture comprises at least 100 ppm PFOA andthe method decreases the PFOA concentration by at least 10⁶ or 10⁷ or10⁸, and in some embodiments up to about 10⁹; wherein the method isconducted in a mobile trailer; wherein the method is conducted in amobile trailer at a PFAS-contaminated site.

Any of the inventive methods may be further defined by, in the overallprocess, or the SCWO portion of the process, can be characterized byconverting a PFAS-concentration of at least 100 ppb PFAS by weight (insome embodiments at least 500 ppb or at least 1000 ppb PFAS) to 1 ppb orless, or 0.1 ppb or less, or 0.01 ppm or less or 7 ppt or less.Alternatively, by converting a PFOA-concentration of at least 100 ppbPFOA by weight (in some embodiments at least 500 ppb or at least 1000ppb PFOA) to 1 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less, or5.0 ppt or less PFOA; in some embodiments in the range of 1 ppm to 5 ppt(part per trillion) PFOA. Alternatively, by converting aPFOS-concentration of at least 100 ppb PFOS by weight (in someembodiments at least 500 ppb or at least 1000 ppb PFOS) to 1 ppb orless, or 0.1 ppb or less, or 0.01 ppb or less, or 5.0 ppt or less PFOS;in some embodiments in the range of 1 ppm to 5 ppt (part per trillion)PFOS. The process can also be characterized by the same levels ofdestruction beginning with a PFAS concentration of 1 ppm or more. Insome embodiments, PFAS-contaminated water comprising at least 1000 pptof at least one (or at least 3 or at least 4 or at least 5 or at least6) compound selected from the group consisting of PFHxA(perfluorohexanoic acid), PFHpA (perfluoroheptanoic acid), PFOA, PFBS(perfluorobutane sulfonate), PFHxS (perfluorohexane sulfonate), PFHpS(perfluoroheptane sulfonate), and PFOS and combinations thereof, treatedby the process is (are) reduced by at least 2 (or at least 3 or at least4 or at least 5) orders of magnitude. In some embodiments,PFAS-contaminated water comprising at least 100 ppt of at least one (orat least 3 or at least 4 or at least 5 or at least 6) compound selectedfrom the group consisting of PFBA (perfluorobutanoic acid), PFPeA(perfluoropentanoic acid), PFHxA, PFHpA, PFOA, 6:2 FTS (6:2fluorotelomer sulfonate), and 8:2 FTS (8:2 fluorotelomer sulfonate) andcombinations thereof, treated by the process is (are) reduced by atleast 2 (or at least 3 or at least 4 or at least 5) orders of magnitudeand/or reduced to 5 ppt (or 1 ppt) or less.

The invention also includes apparatus for destroying PFAS comprising aSCWO reactor and any of the components described herein. The inventionalso includes a system for destroying PFAS comprising a SCWO reactorcomprising a PFAS-containing aqueous mixture. The system may compriseany of the conditions and/or fluids described herein.

Removal of salt is advantageous to prevent clogging in the SCWO reactorand may provide additional advantages such as reduced corrosion andproduction of a PFAS-free aqueous effluent with fewer contaminants.

Leaving the SCWO reactor, the resulting clean water (non-briny) fractioncan optionally be passed through adsorbent media such as activatedcarbon or ion exchange resin and returned to the environment. As withany of the aspects described herein, this pretreatment method may beused by itself or in combination with any of the other aspects or othertechniques described herein.

Various aspects of the invention are described using the term“comprising;” however, in narrower embodiments, the invention mayalternatively be described using the terms “consisting essentially of”or, more narrowly, “consisting of.”

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a water pretreatment system fortreating PFAS-contaminated water prior to passage through a SCWOreactor.

FIG. 2 illustrates a salt separator with inlet and outlet pipesconnected to a collector tube.

FIG. 3 a illustrates a transcritical hydrocyclone with an open channel.

FIG. 3 b illustrates the transcritical hydrocyclone in a nonoperationalmode with a closed channel.

FIG. 3 c shows the diffuser of the transcritical hydrocyclone.

FIG. 3 d shows a cutaway view of the transcritical hydrocyclone.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, PFAS-contaminated water has theconventional meaning. Since the inventive methods are capable ofreducing the concentration of PFAS to less than 5 ppt, the method can beapplied to solutions containing greater than 5 ppt, more typically, atleast 1 ppm. The source of the PFAS-contaminated water can be from soilor surface or underground water in areas subjected to PFAScontamination. These areas can be industrial areas such as theelectronics industry (e.g., wire/cable coatings and semi-conductor boardfabrication), and especially where water-proofing or non-stick coatingshave been applied. Another common source of PFAS-contaminated water isin areas around airfields or firefighting training areas that have beenexposed to AFFF (aqueous film forming foam). Another source can bestorage vessels, typically these sources are accumulated for futuredestruction or disposal. Typically, there will be non-fluorinatedorganic compounds present in PFAS-contaminated water and, especially inAFFF residue, there can be chlorinated or brominated compounds.

Pretreatment

Debris and other solids can be removed from the PFAS-contaminated waterprior destruction of the PFAS. Typically, this can be accomplished byone or a plurality of filtration steps. In some embodiments, a pluralityof filtration steps can be conducted in which increasingly smallerparticles are removed. The filters can be valved so that only one or aseries of filters can be utilized; for example one filter or a set offilters can be cleaned or exchanged while another filter or set offilters continue to operate. Filters can be any type of filter known forfiltering water such as bag filters, cartridge filters, metal screen orsand (preferably silica sand). Alternatively, or in addition,centrifugal separation can be used to remove solids.

The PFAS-contaminated water can be subjected to a softening treatment toremove undesired counterions (typically Ca and Mg) because these foulthe RO membrane. These softening treatments may include one or anycombination of the following: ion exchange resin, lime softening(aqueous calcium hydroxide to precipitate solids); chelating agents (forexample, treatment with EDTA or the like); and reverse osmosis. In anypretreatment, capture of PFAS in pretreatment media should beconsidered. Alternatively, or in addition, compounds such as organicscan be removed by passage through hydrophobic clay to remove separatedand/or emulsified hydrocarbons.

Reverse osmosis (RO) systems can remove or concentrate PFAS from waterstreams. PFAS-free (or PFAS-reduced) water travels through the membranewhile the PFAS and salts are directed to a brine stream. Efficiency ofPFAS removal and throughput is increased by implementing a cascade of ROmembranes. In some embodiments, RO is utilized to increase theconcentration of PFAS by at least 5 times or at least 10 times, and insome embodiments in the range of 5 to 30 times or 5 to 20 times, or 10to 40 times. In some preferred embodiments, the influent to ROpreferably has a total dissolved solids to 1200 ppm or less; however,other systems comprising larger pumps and tighter wound membranes canhandle much higher TDS and achieve effective concentration in accordancewith the present invention, chlorine levels of 0.5 ppm or less morepreferably 0.1 ppm or less, pH between 1 and 12, more preferably between2 and 11, the substantial absence of oil or grease, very low levels ofBa and Si (if present initially, these can be removed in a watersoftening step); a flowrate depending on the scale required, in someembodiments, the RO will be conducted in a range of about 3 to 5 gallons(11 L to 19 L) per minute.

A preferred embodiment of the invention is schematically illustrated inFIG. 1 . PFAS contaminated water entering the system can be subjected tonumerous optional pretreatments including one of more of: filtration(not shown) storage in tank 102, a water softening pretreatment 104, afeed tank 106 connected to a reverse osmosis system 108. Water softeningto replace other cations with sodium cations can be conducted byconventional means such as passage through an ion exchange resin. Thereverse osmosis treatment (described above) produces a permeate 100having PFAS concentrations that are reduced 10×, 100×, 1000×, 10,000× ormore as compared to the PFAS contaminated water entering the system. Insome cases, especially with relatively concentrated PFAS solutionentering the system, the permeate can be subjected to additional ROtreatment to bring the PFAS levels in the permeate down to a low level,such as below 70 ppt, where the water can be released to theenvironment; the retentate from the additional treatments can becombined with the concentrated solution or combined with incoming PFAScontaminated water such as in tank 102.

The retentate 112 typically comprises an aqueous solution having a PFASconcentration that is 10×, 100×, 1000×, 10,000× or more as compared tothe PFAS contaminated water entering the system. Thus, the inventionprovides an energy efficient system in which greater than 90% or 99% or99.9% or more of the PFAS in PFAS contaminated water is completelydestroyed. Thus, in a preferred PFAS destruction process only 10%, oronly 1%, or only 0.1%, or only 0.01% or less of the water in the PFAScontaminated water is heated to SCWO conditions.

The concentrated PFAS water can be passed through optional heatexchanger 114 which can be a tube-in-tube heat exchanger.

The concentrated PFAS water 112 passes into salt separator 116. The saltseparator can have a plurality of zones that operate at differentconditions of temperature or pressure. The tubes can be heated by a tubefurnace that surrounds the tubes. In the case of a plurality of verticaltubes (six shown in FIG. 1 , three upward and three downward) can have arelatively large inner diameter—for example, at least 1.5 cm or at least2.0 cm or at least 1 inch (2.4 cm)—to prevent plugging. At the bottom(with respect to gravity) of each salt separator tube is a largerdiameter container (collector vessel 220), preferably having an innerdiameter of at least 5 cm, or at least 10 cm, or in the range of 5 to 20cm. Preferably the collection vessel includes a diameter that is atleast two times or at least four times larger than the inlet tube. Thecollection vessel can be heated; for example by electrical tape. Thecollector vessel(s) connect the inlet and outlet, preferably have adepth of at least 20 cm, or at least 30 cm, or at least 40 cm, and insome embodiments in the range of 25 to 75 cm. Salt forming in the inlettube falls into the collection vessel where is can be continuously, ormore typically, periodically removed and, if necessary, treated toremove PFAS or other contaminants. Toward the bottom of the collectorvessels there is preferably a valve leading to a drain to remove brineor a briny slurry that collects at the bottom of the collector vessel.Optionally, a pump assembly can be used to evacuate the contents of thecollector vessel at high pressure during operation. In some preferredembodiments, a salt separator tube inlet 222 (carrying fluid into thecollector tube) extends into the collector vessel by at least 5 cm or atleast 10 cm (relative to the outlet into an upward flowing tube); thisenhances downward flow of the saltier fraction into the bottom of thecollector tube forcing the lighter fraction out of the outlet 224. Thecollector vessel(s) may contain baffles to minimize turbulence andmixing near the bottom of the collector vessel(s). Typically, conditionsin the bottom of the collection vessel are subcritical.

The concentrated PFAS water 112 typically enters the salt separator atsubcritical but preferably near supercritical conditions so that thesalt is completely dissolved in the water allowing greater residencetime for salt to fall out of solution and fall into the collectionvessel. Alternatively, the water 112 can enter the salt separator atsupercritical conditions. In the salt separator temperature is increasedso that the solution becomes supercritical and sodium chloride and othersalts precipitate from solution. Conditions (typically temperature) insuccessive zones of the salt separator can be controlled so that thesalt becomes increasingly insoluble as it travels through the saltseparator. In some embodiments, the solution entering the salt separatorcan be below 370° C. and increased in the range of 375 to 450° C. in thesalt separator. Optionally, a fuel, such as an alcohol, could be addedprior to or during the salt separation stage in order to increasetemperature; this may be especially desirable since heat transfer fromthe tube furnaces into the aqueous composition was surprisingly found tobe less than predicted by calculation.

Contact of the briny subcritical phase with the supercritical phase mayallow PFAS to preferentially partition into the supercritical phase;preferably the concentration of PFAS in the briny phase is at least 20mass % less or at least 50 mass % less than in the supercritical phase.Greater than 90 mass % or greater than 95 mass % of the NaCl (or othersalts that are insoluble in supercritical water) can be removed in thesalt separation stage while only 5 mass % or less, or 2 mass % or less,or 1 mass % or less or 0.5 mass % of the PFAS (or organic decompositionproducts) is removed in the briny phase. Different salts precipitate atdifferent temperatures and can be removed at different stages of thesalt separation.

Following the salt separator, the de-salted water can pass through aheat exchanger 118 and then is typically combined with an oxidant 120,such as hydrogen peroxide, prior to introduction into SCWO reactor 144where any remaining PFAS is destroyed. Although in the figure provided,peroxide (or other oxidant) can be added is introduced immediatelybefore the reactor, we more than likely will have the option to add theperoxide at various locations, including upstream of the salt separator.The advantages of adding oxidant in a plurality of locations include 1)minimizing the potential for a hot spot at the location where theperoxide is added, and 2) facilitating destruction of PFAS in the saltseparator. However, a disadvantage of adding peroxide upstream of thesalt separators is that corrosion can be exacerbated. The PFAS-freeeffluent can be passed through heat exchanger(s) such as 118, 114 torecover heat and then stored or passed out of the system as PFAS-freeeffluent 124.

The clean effluent preferably passes back through the second and firstheat exchangers. At any point after the SCWO reactor, the cleaned wateris preferably neutralized, such as by addition of sodium hydroxide.Also, if necessary, the cleaned water can be treated (for example toremove Cr or other metals) prior to disposal or return to theenvironment.

Transcritical Hydrocyclone

In an alternative embodiment, the salt can be removed in a transcriticalhydrocyclone. The transcritical hydrocyclone 30 comprises: a conicalchamber 32 comprising an inlet 34 for introducing supercritical fluidinto the conical chamber tangentially along an inner wall 33 of thecyclone; a top outlet 35 adapted for flow of a supercritical fluid; anexit pipe 36 adapted for flow of a liquid; a cone 37 disposed in theconical chamber adapted such that a channel 38 can be formed between theinner wall of the cyclone and an outer wall of the cone. The cone has abottom opening which is preferably a diffuser. The diffuser has aplurality of openings 41. The openings preferably have a combined openarea that is at least two times or at least three times or more than thecross-sectional area of the exit pipe. In some preferred embodiments,the holes in the diffuser can be oriented at an angle counter to thedirection of flow of the cyclone; for example, if the cyclone is in theclockwise direction, the holes are oriented at an angle that is counterclockwise. Preferably, the diffuser extends into the exit pipe.

Typically, the hydrocyclone includes a fluid heat exchanger (not shown)that cools outer wall 42 of the conical chamber; typically water is thecoolant. The heat exchanger is disposed on the conical section and mayextend down to the exit pipe and as far down as the separator.

During operation, the salt-containing PFAS aqueous supercriticalcomposition enters through the inlet 34 and flows tangentially along theinner wall of the conical chamber. The cone forms a conical channel 38.Preferably, the cone can be adjusted to change the cross-sectional areaof the conical channel. For example, the cone can be attached to outletpipe 35 via a threaded connection with a lock nut and/or washer.Reducing the area of the flow channel increases the velocity of thefluid through the channel.

The inner wall 33 is sufficiently cool that a small subcritical phaseforms adjacent to the wall 33. Salt in the supercritical phase isrelatively dense and the centrifugal force drives the salt to the innerwall 33 where it passes into the subcritical phase on the inner wall; insome cases, the subcritical phase forms droplets on the wall. Thesubcritical brine phase drains downward into the exit pipe. At thebottom of the exit pipe can be a two-phase gravity separator equippedwith a level transmitter controller and control valve.

Supercritical Water Oxidation (SCWO)

PFAS-containing water is preferably heated prior (typically immediatelyprior) to entering the reactor. Heat from the reactor is used to heatwater entering the reactor. The use of a heat exchanger makes theprocess more energy efficient, compact and extends service life of thereactor. A tube-in-tube heat exchanger is especially desirable. PFAS aredestroyed and converted to carbonates, fluoride salts and sulfates. Thedevice can be designed for 1) stationery applications or 2)transportation to a site. The stationery configuration can be employedat a permanent processing plant such as in a permanently installed waterfacility such as city water treatment systems. The portable units can beused in areas of low loading requirement where temporary structures areadequate. A portable unit is sized to be transported by a semi-truck orsmaller enclosed space such as a trailer or shipping container. Thedesign is adaptable to processing other organic contaminants bymodifying operational parameters but without modification of the device.

A preferred SCWO reactor design is a continuous or semi-continuoussystem in which the (typically pre-treated) PFAS-containing aqueoussolution is passed into a SCWO reactor. Because solids may form in theSCWO reactor, it is desirable for the reactor to slope downward so thatsolids are pulled by gravity downward and out of the reactor. In someembodiments, the flow path is straight and vertical(0°) with respect togravity; in some embodiments, the reactor is sloped with respect togravity, for example in the range of 5 to 70° (from vertical) or 10 to50° or 10 to 30° or 10 to 20° and can have a bend so that flow moves ina reverse direction to provide a compact device in which flow isconsistently downward with respect to gravity. Preferably, the reactorvessel is a cylindrical pipe formed of a corrosion resistant material.Desirably, the pipe has an internal diameter of at least 1 cm,preferably at least 2 cm and in some embodiments up to about 5 cm.

Flow through the components of the SCWO apparatus at supercriticalconditions should be conducted under turbulent flow (Re of at least2000, preferably in the range of 2500 to 6000). Effluent from the SCWOreactor can flow into a salt separator under supercritical conditions.

The SCWO system operates by raising the feed temperature and raising thefeed pressure. The increased pressure can be due solely due to theheating (which is preferable) or can be further increased via acompressor or a high pressure (reciprocating) pump. The temperature isincreased by: application of heat through the conduit (in the case of acontinuous reactor) or through the reaction chamber in the case of abatch reactor, and/or by the addition of fuels such as alcohols orhydrocarbons that will be oxidized to generate heat in solution.Supercritical conditions are maintained for the oxidation; conditionswithin the reaction conduit or reaction chamber are preferably in therange of 374° C.-700° C. and at least 220 bar, more preferably 221-300bar. In some embodiments, temperature in the SCWO reactor is maintainedat 500° C. or more, or 600° C. or more and in the range of 500 to 650°C., or 600 to 675° C.

Oxidants

The two tested feedstocks of reactant oxygen used in supercritical wateroxidation for destruction of PFAS are oxygen gas (O₂) and hydrogenperoxide (H₂O₂). In addition to, or alternative to, these two chemicalspecies, other reactant oxygen sources or oxidizing agents could beadded to destroy PFAS in the oxidation reactor. Other oxidants maycomprise oxyanion species, ozone, and peroxy acids.

The preferred oxidant is hydrogen peroxide which can be added in excess(for example an excess of at least 50% or at least 100% or in the rangeof 50% to 300% excess) and the excess hydrogen peroxide reacting to formdioxygen and water.

Fuels

At start up, the SCWO apparatus requires heating such as by externalflame or resistive heating. Unless the reactive solution comprises highconcentrations of PFAS or other organics, external heating is alsoneeded during operation. As an alternative, or in addition to externalheating of the SCWO apparatus, heat can be provided by the oxidation offuels such as alcohols. Preferred fuels include methanol, ethanol,propanol (typically isopropanol), or combinations of these.

Handling The Fluorine By-Products From Destruction Of PFAS

The corrosive effluent from the SCWO reactor containing aqueous HF athigh temperature (for example, around 600° C.) can flow into a mixingpipe. Cooling water, typically containing hydroxy salts, can be fed intoa mixing pipe where it mixes with the corrosive effluent. The cooledeffluent contains dissolved fluoride salts such as NaF.

Post Treatment

Since the SCWO process destroys essentially all of the PFAS, the treatedeffluent can be safely released back into the environment. In someembodiments, at least a portion of the effluent is evaporated into theair. The vapor generated will typically be at 100% humidity because ithas been cooled and in equilibrium with the aqueous phase. However, thereason that there is a vapor stream is due to the carbon dioxide formedas a reaction byproduct as well as excess oxygen to ensure completeoxidation. Feeds (such as PFAS-spiked distilled water samples) thatcontain relatively little organic vapor generate very little (sometimesnot measurable) vapor. This is safe since the PFAS has been destroyedand any remaining contaminants (such as metals, NaF, etc.) tend to havevery high vapor pressure so that they do not evaporate with the water.Precipitates such as fluoride salts can be filtered or centrifuged fromthe effluent. PFAS-free effluent can be passed through the heatexchanger where the effluent is cooled by the PFAS-contaminated waterflowing into the reactor. If necessary, the effluent may be subjected totreatments such as reverse osmosis and/or other treatments (ion exchangeresins and other adsorptive media (Metsorb™), etc.) to remove metals orother contaminants prior to release or disposal of the effluent.

Passivation of Interior Surfaces of Salt Separator or SCWO Reactor

The interior surfaces of the apparatus can be coated with corrosionresistant materials such as platinum aluminide, B4C (boron carbide), SiC(silicon carbide), TaC (tantalum carbide), WC (tungsten carbide), metalfluorides such as YF3 (yttrium fluoride), YN (yttrium nitride), LaF3(lanthanum fluoride), LaN (lanthanum nitride), YbN, YbF3, or anylanthanide nitride or lanthanide fluoride, HfN (hafnium nitride), CeN(cerium nitride), CeF3 (cerium fluoride), TaN (tantalum nitride), Ta(tantalum), TaF (tantalum fluoride), ZrN (zirconium nitride), ZrF(zirconium fluoride), WN (tungsten nitride), or combinations thereof.The coating material resists corrosion under conditions of theprocess—supercritical water, typically HF, oxidant; thus protecting theunderlying metal. A preferred coating material is a chromium oxidecoating (preferably comprising at least 80 wt % or at least 90 or 95 wt% chromium oxide that is compatible with fluids with pH levels of 0.1 to11 and is stable up to at least 2,300° F. (1260° C.). It is not affectedby the low levels of HF content we expect in our process. The coatingcould be made, for example, by wash coating the apparatus walls with aslurry of chromium oxide and then baking at 752° F. to 975° F. (400° C.to 525° C.). Multiple cycles of coating and/or baking are desirable toensure minimal to no porosity.

Alternatively or in addition, corrosion can be reduced by use of asacrificial electrode or impressed current cathodic protection.

Mobile Units

One example of a mobile unit is one that can be transported on (andpreferably operated within) a trailer. For example, the system can betransported (and optionally operated) on a trailer having dimensions of29 feet (8.8 m) in length or less, 8 ft 6 in (2.6 m) or less width, and13 ft 6in (4.1 m) height or less. These dimensions define preferred sizeof a mobile system, although workers in this area will understand thatother dimensions could be utilized in a mobile unit.

Example

Reverse Osmosis— 364 kg of well water was spiked with PFAS to aconcentration of 2400 ug/L. The spiked sample was passed through a watersoftener (measured PFAS concentration showed no change in the watersoftening step). The softened PFAS water was run through a ReverseOsmosis system for 160 minutes until the retentate mass was 38.5 kg. Theresults, where % efficiency=(C_(feed)−C_(perm)/C_(feed))×100, is shownin the Table below:

RO Feed Concentrate - Permeate - Analytes Tank End of Run End of Run %Efficiency TOTAL PFAS 2,383.88 14,416.58 22.86  99.0% PFBA 256.991,535.07 1.40 99.5% PFPeA 221.03 1,574.07 1.39 99.4% PFHxA 238.421,526.35 1.71 99.3% PFHpA 211.28 1,425.79 1.97 99.1% PFOA 502.523,239.44 5.70 98.9% PFDA 209.10 1,232.77 3.13 98.5% PFUnA 235.041,212.24 2.69 98.9% PFDoA 165.07 783.55 2.75 98.3% PFTrDA 0.22 1.66 ND100.0% pFTeDA 0.18 1.43 ND 100.0% PFHxS 0.03 0.49 ND 100.0% PFHpS 1.475.10 ND 100.0% PFOS 236.63 1,439.95 1.12 99.5% PFNS 0.42 0.61 ND 100.0%4:2FTS 0.37 1.17 ND 100.0% 6:2FTS 103.94 431.45 0.75 99.3% 8:2FTS 0.531.84 ND 100.0%

The testing showed that PFAS was not removed by filters or softening.High PFAS removal efficiencies were accomplished along with a ten-foldincrease in PFAS concentration in the retentate. Surprisingly, aprecipitate was observed in the retentate which could be dissolved byreducing pH/addition of acid.

What is claimed:
 1. A method of destroying PFAS, comprising: providingan aqueous solution comprising water and PFAS; subjecting the aqueoussolution to reverse osmosis to produce a clean water fraction and abriny concentrated fraction in which the PFAS concentration is at least50% greater than the aqueous solution; preheating the briny concentratedfraction in a heat exchanger to form a preheated concentrated fractionthat is at subcritical conditions; passing the preheated concentratedfraction into a pre-reactor where the briny concentrated fraction isconverted to supercritical conditions at a first temperature causingsodium chloride to precipitate; removing at least a portion of thesodium chloride to produce a brine-reduced fraction; passing thebrine-reduced fraction to a reactor where the fraction is subjected tooxidation under supercritical conditions wherein the concentration ofoxidant and/or temperature is higher than in the pre-reactor; producinga clean hot water solution having a concentration of PFAS that is atleast 90% less than the aqueous solution; and, wherein a fuel oroxidizer is added to the pre-reactor; and further comprisingtransferring heat from the clean hot water solution to the aqueoussolution in the heat exchanger in the preheating step.
 2. The method ofclaim 1 the fuel or oxidizer comprises an alcohol.
 3. The method ofclaim 1 the pre-reactor comprises a trans-critical hydrocyclone.
 4. Themethod of claim 1 the brine-reduced fraction passes through a heatexchanger and then combined with hydrogen peroxide prior to beingintroduced into a SCWO reactor.
 5. The method of claim 1 wherein theaqueous solution comprising water and PFAS has a first volume; wherein10% or less of a first volume is subjected to supercritical conditions;and wherein, in said method, at least 95% of the PFAS in the firstvolume is destroyed in supercritical conditions.
 6. The method of claim1 wherein the method is carried out in a trailer or a shippingcontainer.
 7. A method of destroying PFAS, comprising: providing anaqueous solution comprising water and PFAS; treating the aqueoussolution to reduce its volume to form a reduced volume PFAS solutionhaving a first concentration of PFAS; adding hydrogen peroxide to thereduced volume PFAS solution wherein the hydrogen peroxide is added inexcess of that needed to destroy the PFAS; passing the reduced volumePFAS solution into a SCWO reactor and subjecting the reduced volume PFASsolution to supercritical water oxidation; and producing a cleaneffluent having a concentration of PFAS that is more than 100,000 timesless than the first concentration of PFAS: and wherein flow through theSCWO reactor is turbulent flow with an Re of at least
 2000. 8. Themethod of claim 7 wherein the clean effluent comprises 5 ppt of less ofPFAS.
 9. The method of claim 7 comprising producing a clean effluenthaving a concentration of PFAS that is more than 1,000,000 times lessthan the first concentration of PFAS.
 10. The method of claim 7comprising producing a clean effluent having a concentration of PFASthat is more than 10,000,000 times less than the first concentration ofPFAS.
 11. The method of claim 7 wherein the clean effluent is treatedwith a solution comprising NaOH, LiOH, or KOH.
 12. A method ofdestroying PFAS, comprising: providing an aqueous solution comprisingwater and PFAS; treating the aqueous solution to reduce its volume toform a reduced volume PFAS solution having a first concentration ofPFAS; adding hydrogen peroxide to the reduced volume PFAS solutionwherein the hydrogen peroxide is added in excess of that needed todestroy the PFAS; passing the reduced volume PFAS solution into a SCWOreactor and subjecting the reduced volume PFAS solution to supercriticalwater oxidation; and producing a clean effluent having a concentrationof PFAS that is more than 100,000 times less than the firstconcentration of PFAS; and wherein the reduced volume PFAS solution ismixed with a solution comprising 30 to 50 wt % H2O2 at a weight ratio ofbetween 30:1 and 70:1 wt % PFAS solution.
 13. The method of claim 7wherein the reduced volume PFAS solution is passed through the SCWOreactor with a residence time of 20 sec or less.
 14. The method of claim7 wherein, after start-up, no external heating is needed for the SCWOreactor.
 15. The method of claim 7 wherein the method is conducted in amobile trailer at a PFAS-contaminated site.
 16. The method of claim 7wherein SCWO reactor has an internal ceramic coating and wherein theceramic comprises: B4C (boron carbide), SiC (silicon carbide), TaC(tantalum carbide), WC (tungsten carbide), metal fluorides such as YF3(yttrium fluoride), YN (yttrium nitride), LaF3 (lanthanum fluoride), LaN(lanthanum nitride), YbN, YbF3, or any lanthanide nitride or lanthanidefluoride, HfN (hafnium nitride), CeN (cerium nitride), CeF3 (ceriumfluoride), TaN (tantalum nitride), Ta (tantalum), TaF (tantalumfluoride), ZrN (zirconium nitride), ZrF (zirconium fluoride), WN(tungsten nitride), or combinations thereof.
 17. A method of destroyingPFAS, comprising: providing an aqueous solution comprising water andPFAS; treating the aqueous solution to reduce its volume to form areduced volume PFAS solution having a first concentration of PFAS;adding hydrogen peroxide to the reduced volume PFAS solution wherein thehydrogen peroxide is added in excess of that needed to destroy the PFAS;passing the reduced volume PFAS solution into a SCWO reactor andsubjecting the reduced volume PFAS solution to supercritical wateroxidation; and producing a clean effluent having a concentration of PFASthat is more than 100,000 times less than the first concentration ofPFAS; and wherein the SCWO reactor comprises a flow channel having aninternal diameter of at least 1 cm that slopes downward with respect togravity.
 18. The method of claim 17 wherein flow through the SCWOreactor is turbulent flow with an Re of at least
 2000. 19. The method ofclaim 7 wherein the hydrogen peroxide is added in at least 50% excess.